Year ‍13 Mechanics Vertical Circular Motion demos:


  • The purpose of this experiment is to see the relationship between forces for an object undergoing vertical circular motion


  • Force meter (ideally digital)
  • Mass on a string
  • Bucket (strong + cheap / weak)
  • Water


Part 1: Spinning bucket with a difference:
  • Go out side with a bucket filled with water. As a student to have the bucket upside down, without the water coming out.
  • Students will naturally spin the bucket.
  • Ask the student spinning the bucket where are the forces the greatest? - if you use a cheap bucket with alot of water in it, chances are will break - a good and fun part of the demo. As long as it gets above the person's head, it will always break at the bottom - a good talking point as to why....
  • You can also get the student to extra fast - ask what happens (the water is more compressed into the bucket)
  • You can get the student to try and go as slow as possible - ask / see what happens when you go too slow - the water falls out

Part 2 mass on string:
  • Get a mass on a string (I use a tennis ball with a string through it, but a rubber bung etc. would work fine).
  • Connect the mass to a force meter (ideally digital [pasco PS-2104] )
  • Measure and record the weight force of the mass
  • Ask the students what force (tension in the string) they expect when the ball is at the top of its motion, and when the ball is at the bottom of its motion.
  • Then prove this, by spinning the ball in a vertrical circle, at the slowest possible velocity for the ball to maintain its circular motion. Students will see that the tension force in the string is zero when the ball is at the top of its motion, and the tension in the string is much more that the weight force of the ball at the bottom of its motion.
    • The answer is because the weight force of the ball (or bucket filled with water) at the top is equal to / provides the centripetal force component at the top, so no tension force is required. However, when the ball (bucket) is at the bottom of its motion, weight force is pushing down, but a centripetal force is still required, so an upward force comprising of the weight force + the centripetal force is required. Hence why the bucket breaks at the bottom of the swing.


  • Model graph:
Graph measured by digital force meter showing: no tension force (F=0N) when the ball was at the top of its motion, and the tension force = max ~4N when the ball was at the bottom of its motion

Working, to determine the centripetal force (using the time between two peaks from the above graph), and the weight force from the digital force meter when the ball was not moving (Fw =0.82N).

Watch on physicstube.co.nz
Watch on physicstube.co.nz
For more circular motion videos, select the circular motion video catergy on www.physcistube.co.nz

Pitfalls / causes of error

  • .....

References / other comments:

  • ......